1. Field of the Invention
The present invention relates to driving light source blocks of a display apparatus such as a liquid crystal display (LCD).
2. Description of the Related Art
A liquid crystal display apparatus includes an LCD panel displaying images by controlling the transmissivity of liquid crystal. A backlight unit can be placed behind the LCD panel to provide light to the LCD panel.
The LCD panel may include an array substrate having pixel electrodes and thin-film transistors (TFTs) electrically connected to the pixel electrodes, a color filter substrate having a common electrode and color filters, and a liquid crystal layer disposed between the array substrate and the color filter substrate. The orientation of the liquid crystal molecules in the liquid crystal layer is controlled by controlling the electric field between the pixel electrodes and the common electrode and thus controlling the transmission of light passing through the liquid crystal layer.
When the liquid crystal's transmissivity is maximized, the LCD panel displays a white image having a high luminance. When the transmissivity is minimized, the LCD panel must display a black image having a low luminance. However, it is difficult to cause the liquid crystal molecules to block all light for low luminance images. Light may leak out of the LCD panel, making it difficult for example to display black images. Consequently, the contrast ratio (CR) of the image displayed on the LCD panel becomes reduced.
Recently, the CR has been increased using a local dimming method. This method is applicable if the backlight unit includes a plurality of individually controlled light source blocks. If an image includes a dark portion, the corresponding light source blocks (i.e. the light source blocks located directly opposite to the dark portion) are dimmed to reduce the black portion's luminance. The CR is consequently increased.
In the local dimming method, the luminance of the light source blocks is controlled based on the pixel data, i.e. the data that define the color of each picture element (pixel) in the image. Each pixel of the image (“image pixel”) corresponds to one or more pixels (“device pixels”) of the LCD panel. More particularly, each image pixel corresponds to those device pixels that are used to display the image pixel. Each light source block corresponds to a device pixel block which is the device pixels region directly opposite to the light source block. The device pixels block in turn corresponds to an image pixels block which consists of image pixels displayed by the device pixels block. In the local dimming method, the luminance of the light source blocks is controlled based on the pixel data, and the pixel data (the image pixels) are then adjusted (compensated) for the dimming as needed to provide the desired image (e.g. the adjustments may be done to increase the transmissivity of some of the device pixels corresponding to dimmed light source blocks). Generally, the luminance of each light source block is set based on a mean value or a maximum value of the pixel data corresponding the light source block (i.e. the pixel data for the image pixel block corresponding to the light source block). However, when the luminance of the light source blocks is set based on the mean value of the corresponding pixel data, then bright images are displayed well, but dark images look too dark. On the other hand, when the luminance of the light source blocks is set based on the maximum value of the corresponding pixel data, and at least one pixel has a large luminance as defined by the pixel data, then the light source blocks are not effectively dimmed and therefore the local dimming process is not effective.